An actuator unit for an injection valve of an internal combustion engine of a vehicle typically comprises a structural element which is formed as a stack and which has a plurality of electrode layers and a plurality of material layers reacting to the application of an electrical field, each material layer being arranged between two of the electrode layers. Such a structural element of layers of material layer and electrode layer which are stacked one above the other and alternately to one another is designated in general as a stack. The currently best known electronic structural element of this type is a stack which is designated in general as a piezo-actuator and which is employed as an actuation element in injection valves of the most diverse possible engine types for motor vehicles. In this piezo-actuator, the material layers are ceramic layers.
Conventionally, such a stack, as seen in a top view, has a rectangular or square cross section. The stack is typically contacted electrically on two circumferential sides lying opposite one another. So that this can be carried out in a technologically careful way, the electrode layers are, for example, designed geometrically such that only every second electrode layer extends laterally as far as one of the two circumferential sides, whereas the in each case other electrode layers do not extend as far as this one circumferential side.
The same applies correspondingly to the other circumferential side of the stack. Furthermore, fully active stacks, as they may be referred to, are known, in which the electrode layers and the material layers have the same area, with the result that all the electrode layers extend in each case up to the circumferential sides lying opposite one another.
In both cases, the electrical contacting of the electrode layers takes place via two external electrodes which are connected electrically to respective electrode layers, in general, on at least one circumferential side of the structural element and, typically, on two circumferential sides lying opposite one another. The type of contacting depends in this case on whether the stack is fully active or not.
Where a fully active stack is concerned, passivation is applied to the respective circumferential sides in order to generate the external electrodes. Subsequently, every second electrode is exposed by means of subtractive methods by pits or recesses being generated in the passivation. An electrically conductive material, such as, for example, a conductive adhesive, is thereupon applied to the passivation, penetrates into the pits or recesses and makes electrical contact with the electrode layers. This procedure entails some disadvantages.
For the passivation, it is necessary to have a material which can subsequently be removed in a targeted way and is temperature-resistant up to 190° C. These requirements necessitate the material group of polyimides which, however, contain solvents harmful to health. During manufacture, therefore, these solvent require measures for protecting the health of employees, thus making the processes more complicated. The same applies to the conductive adhesive by which further contacting takes place after the exposure of the electrodes.
The finished structural element is surrounded by a tube spring which is typically composed of a metal. The tube spring serves to prestress the structural element stack when the actuator unit is in operation. Furthermore, the tube spring serves for providing sufficient performance for the deflected structural element stack. As insulating material between the tube spring and the external electrodes of the structural element stack, a layer, for example of silicone, which covers at least the external electrodes, is provided on the outer circumference of the structural element stack.
EP 1 835 553 A1 discloses a method for producing a piezoelectric actuator, in which the contacting structure is applied to an insulating layer which is applied subtractively or additively to corresponding stack circumferential regions.